INTRODUCTION 635 



A. Introduction 



1. External and Internal Respiration 



Respiration consists of two phases: (1 ) external and (2) internal. External 



respiration enables the organism to acquire oxygen from its external environ- 

 ment and to discharge carbon dioxide into this environment. Internal respira- 

 tion is the utilization of oxygen and the elimination of carbon dioxide by the 

 cells and tissues of the organism. The formation of the structural mechanisms 

 related to external respiration, in many vertebrates, is associated intimately 

 with buoyancy functions. The development of external respiratory and buoy- 

 ancy mechanisms is discussed in this chapter. 



2. Basic Structural Relationships Involved in External 



Respiration 



a. Cellular Relationships 



In effecting external respiration, it is necessary for blood capillaries to come 

 into a close relationship with a moist or watery medium containing sufficient 

 amounts of oxygen and a lowered content of carbon dioxide. The mechanisms 

 permitting this relationship vary in different vertebrates. In lower vertebrates, 

 blood capillaries in the gills or in the skin are brought near the watery medium 

 containing oxygen, while, in higher vertebrates, lungs are used for this purpose. 

 In lower vertebrates, an epithelial layer of cells is always interposed between 

 the blood stream and the oxygen-containing fluid. Small amounts of mesen- 

 chyme or connective tissue may interpose also (fig. 299B & C). However, in 

 the air capillaries of the lungs of birds (fig. 307C) and in the air cells (alveofi) 

 of mammalian lungs (figs. 299A; 309G), the surrounding blood capillaries 

 may be exposed intimately to the air-fluid mixture containing oxygen, and the 

 barrier of epithelium between the blood capillaries and the air mixture may 

 be greatly reduced if not entirely absent. 



b. Sites or Areas Where External Respiration Is Accomplished 



External respiration is achieved in various areas in the embryos and adults 

 of different vertebrate species. In the early shark embryo, external gill fila- 

 ments, attached to the pharyngeal area, serve as a mechanism for effecting 

 external respiration (fig. 299D), whereas, in the chick and reptile embryo, 

 allantoic contacts with surface membranes of the egg are important (fig. 299E). 

 In the frog tadpole, the flattened tail region is a factor, as well as the presence 

 of gills and lungs associated with the pharyngeal area. The embryos of higher 

 mammals utilize allantoic-placental relationships for this phase of respiration 

 (see Chap. 22). Similarly, in adult vertebrate species, various areas of the 

 body are used as respiratory mechanisms, such as a moist skin (fig. 299B), 

 gills, lungs, vascular villosities, or papillae (fig. 299F). The skin is most im- 



